1. QEC: Decoherence

Decoherence: The Enemy

Quantum states are fragile. Interaction with the environment (heat, radiation, magnetic fields) causes the quantum information to leak out.

This process is called Decoherence.

• T1 (Relaxation): The time it takes for $|1\rangle$ to decay to $|0\rangle$.
• T2 (Dephasing): The time it takes for the relative phase ($lpha|0\rangle + eta|1\rangle$) to scramble.

Without error correction, decoherence limits the depth of our circuits.